The present invention relates to apparatus for and a method of reducing cosmic-ray background interference for improved neuron coincidence counting. More specifically, the present invention utilizes localized neutron coincidence veto (xe2x80x9cLCVxe2x80x9d) to eliminate the counting of cosmic-ray generated background neutrons which have small multiplicity.
Waste materials contaminated with small amounts of plutonium and other transuranic isotopes represent a major disposal problem. The higher the level of, for instance, plutonium, the more expensive the disposal (i.e., storage) cost. Enormous sums have been spent on long term waste storage sites, such as the Waste Isolation Pilot Plant in New Mexico for plutonium contaminated material.
As the neutrons emitted by plutonium can penetrate through the matrix materials of the boxes, drums and crates that are used to hold contaminated waste, passive neutron counting is the most effective method of measuring plutonium in bulk waste containers. To obtain a quantitative relationship between the neutron emission rate and the mass of the plutonium in a given volume of waste material (e.g. in a drum), it is necessary to count the time-correlated (i.e., coincidence) neutrons from the waste to determine the spontaneous fission rate. Detectors large enough to hold 200 liter drums of waste, in which thermal neutron counters are embedded in polyethylene walls, have been used for this purpose. Typically, such detectors will completely surround the waste container, for high counting efficiency to make the coincidence counting practical.
The primary limitation to the sensitivity (i.e., lower detectability limit) of the above described detection method is background interference caused by cosmic rays in the neutron counter and its immediate vicinity. The primary source is the carbon in the polyethylene detector body and adjacent polyethylene shield. Cosmic rays from space react with this carbon to create bursts of neutrons that interfere with the neutron coincidence counting of the sample.
Cosmic ray spallation (i.e., a nuclear reaction in which the energy of each incident particle is so high that more than two or three particles are ejected from the target nucleus) events create multiple neutrons detected in the neutron counter with a large range of multiplicity events. The large multiplicity events (i.e., more than three neutrons per fission) in the background can be separated from plutonium spontaneous fission by virtue of their high multiplicity. Spontaneous fission events from plutonium emit only a few neutrons (i.e., 1-5) per fission event, and only a couple of these are typically detected. Multiplicity sorting and statistical testing have been used to separate the cosmic ray initiated multiplicity events from the plutonium origin spontaneous fission neutrons. However, this is not a complete solution to the interference problem, as about 60% of the cosmic ray background neutrons have only a small multiplicity that is the same as for the plutonium neutrons. Multiplicity sorting and statistical testing are described in, respectively: M. S. Krick, xe2x80x9cThermal Neutron Multiplicity Counting of Samples With Very Low Fission Rates,xe2x80x9d Los Alamos National Laboratory report LA-UR-97-2649 (1997); and D. H. Beddingfield and H. O. Menlove, xe2x80x9cStatistical Data Filtration in Neutron Coincidence,xe2x80x9d Los Alamos National Laboratory report LA-12451-MS (November 1992). The general problem of cosmic ray spallation interference with low activity plutonium measurements using neutron coincidence counting is discussed in H. O. Menlove and G. W. Eccleston, xe2x80x9cHigh-Sensitivity Measurements for Low-Level Tra Wastes Using Advanced Passive Neutron Techniques,xe2x80x9d Los Alamos National Laboratory report LA-UR-92-2563 (1992).
In addition to cosmic ray spallations from the carbon in the polyethylene detector body and adjacent shield, metals, such as iron in the waste and containers, are an additional source of cosmic ray spallation neutrons that limit plutonium detection sensitivity. These spallation events in the metal often generate high multiplicity neutrons that can be eliminated by the truncated multiplicity technique (as referenced in Krick, supra) or by the statistical data filter technique (as referenced in Beddingfield, et al., supra). However, there is a number (about half) of these spallation events that only produce a count of two neutrons (i.e., a doubles event). The number of counts with high multiplicity can be used to predict (by extrapolation) the number of doubles counts. This method is accurate to about 2-3%, depending on the counting statistics. However, counting times of approximately 15 minutes are required to obtain adequate statistics for high multiplicity counts. The weight of the metal in the container can also be used to predict the coincidence background. However, this method has a large statistical fluctuation, which limits the plutonium detector sensitivity.
It is an object of the present invention to more accurately measure plutonium and other transuranic materials in waste materials.
It is another object of the present invention to measure plutonium in waste to a lower level of detection and, thus, save on the costs associated with storage and/or transportation of such waste.
It is another object of the present invention to reduce the counting of cosmic ray generated background neutrons having a small multiplicity.
It is yet another object of the invention to separate plutonium source neutrons from cosmic ray background neutrons to improve the accuracy and sensitivity of sorting nuclear wastes. Such improved accuracy will reduce the costs associated with storage of such wastes.
It is yet another object of the present invention to eliminate the small cosmic-ray multiplicity events which escape the above described truncation method.
It is yet still another object of the present invention to improve plutonium measurement sensitivity by eliminating from the counting many of the cosmic ray neutrons that are born in the waste being sample.
This invention relates to both the apparatus and method for increasing the sensitivity of measuring the amount of radioactive material in waste by reducing the interference caused by cosmic ray generated neutrons. The apparatus includes: (a) a plurality of neutron detectors, each of the detectors including means for generating a pulse in response to the detection of a neutron; and (b) means, coupled to each of the neutrons detectors, for counting only some of the pulses from each of the detectors, whether cosmic ray or fission generated. The means for counting includes a means that, after counting one of the pulses, vetos the counting of additional pulses for a prescribed period of time. The prescribed period of time is between 50 and 200xcexcs. In the preferred embodiment the prescribed period of time is 128xcexcs. The veto means can be an electronic circuit which includes a leading edge pulse generator which passes a pulse but blocks any subsequent pulse for a period of between 50 and 200xcexcs. Alternately, the veto means is a software program which includes means for tagging each of the pulses from each of the detectors for both time and position, means for counting one of the pulses from a particular position, and means for rejecting those of the pulses which originate from the particular position and in a time interval on the order of the neutron die-away time in polyethylene or other shield material. The neutron detectors are grouped in pods, preferably at least 10. The apparatus also includes means for vetoing the counting of coincidence pulses from all of the detectors included in each of the pods which are adjacent to the pod which includes the detector which produced the pulse which was counted.
The method for increasing the sensitivity of measuring the amount of radioactivity in waste by reducing the interference caused by cosmic ray generated neutrons includes: (a) providing a plurality of neutron detectors, each of the detectors including means for generating a pulse in response to the detection of a neutron; (b) counting only some of the pulses from each of the detectors; and (c) vetoing the counting of coincidence pulses from each of the detectors for a prescribed period of time after the counting.